[Bf-blender-cvs] [6cdfd60] hair_system: Fix for restitution force scaling: This has to use the overall timestep size, since collision only happens once during this step.

[Lukas Tönne]

Commit: 6cdfd6013d27cebf9d872639cf3ec090e260c6cf
Author: Lukas Tönne
Date:   Fri Aug 8 12:54:39 2014 +0200
Branches: hair_system
https://developer.blender.org/rB6cdfd6013d27cebf9d872639cf3ec090e260c6cf

Fix for restitution force scaling: This has to use the overall timestep
size, since collision only happens once during this step.

Otherwise the simulation explodes due to forces being a factor
num_force_steps too large.

===================================================================

M       source/blender/hair/intern/HAIR_solver.cpp

===================================================================

diff --git a/source/blender/hair/intern/HAIR_solver.cpp 
b/source/blender/hair/intern/HAIR_solver.cpp
index 81f1217..3358f47 100644
--- a/source/blender/hair/intern/HAIR_solver.cpp
+++ b/source/blender/hair/intern/HAIR_solver.cpp
@@ -313,7 +313,7 @@ static void do_damping(const HairParams &params, const 
SolverForces &forces, flo
        }
 }
 
-static void do_collision(const HairParams &params, const SolverForces &forces, 
float time, float timestep, const SolverTaskData &data, const PointContactCache 
&contacts)
+static void do_collision(const HairParams &params, const SolverForces &forces, 
float time, float timestep, float restitution_scale, const SolverTaskData 
&data, const PointContactCache &contacts)
 {
        int start = data.startpoint;
        int end = start + data.totpoints;
@@ -345,7 +345,7 @@ static void do_collision(const HairParams &params, const 
SolverForces &forces, f
                        /* estimate for velocity change to prevent collision 
(3.2, (8)) */
                        float3 dv_a = dot_v3v3(info.restitution * (obj_v0 - v0) 
+ (obj_v1 - v0), info.world_normal_body) * info.world_normal_body;
                        
-                       point->force_accum = point->force_accum + dv_a / 
timestep;
+                       point->force_accum = point->force_accum + dv_a * 
restitution_scale;
 //                     Debug::collision_contact(point->cur.co, point->cur.co + 
point->force_accum);
                }
        }
@@ -359,6 +359,12 @@ void Solver::do_integration(float time, float timestep, 
const SolverTaskData &da
        int totcurve = data.totcurves;
        /*int totpoint = data.totpoints;*/
        
+       /* Note: the restitution scale is determined by the overall timestep,
+        * since collisions are only generated once during this interval.
+        * Otherwise restitution forces would be num_force_steps times too large
+        */
+       float restitution_scale = 1.0f / timestep;
+       
        for (int step = 0; step < totsteps; ++step) {
                
                /* clear forces */
@@ -412,7 +418,7 @@ void Solver::do_integration(float time, float timestep, 
const SolverTaskData &da
                        
                }
                
-               do_collision(m_params, m_forces, time, dt, data, contacts);
+               do_collision(m_params, m_forces, time, dt, restitution_scale, 
data, contacts);
                
                /* integrate */
                {

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